Added mc-hide-unmatched-lines mode

This minor mode when enabled hides all lines where no cursors (and
also hum/lines-to-expand below and above) To make use of this mode
press "C-'" while multiple-cursor-mode is active. You can still
edit lines while you are in mc-hide-unmatched-lines mode. To leave
this mode press "<return>" or "C-g"
Added testcase for mc-hide-unmatched-lines-mode
Added description of mc-hide-unmached-mode to Readme